Didn't read the article, but I did grep it for unicode and utf-8 and found no matches, so this might also be related. Twitter lets you tweet 140 characters regardless of the bit width of those characters in. For Japanese, I think almost all characters take two bytes in utf-8. As such, given the same number of tweets, the bandwidth usage is approx 2X. Twitter also seems much more useful in ideogram languages as 140 ch…
The vast majority of words in Chinese and Japanese are not single characters. In fact, a very large portion of the characters cannot be used by themselves, at least in modern Japanese. Japanese is also heavily reliant on two other writing systems which are far less space-efficient. A single character of either hiragana or katakana represents only a single syllable of sound (such as か(ka),ぽ(po),し(shi),etc.). Unless your tweets were just very long noun phrases without any grammar pertaining only to things that can be written with Chinese characters (which means no modern foreign words), less efficient writing systems would be needed.
So, while Japanese may be slightly more compact due to the lack of spaces and the ability to assign a large number of sounds to a single Chinese character (such as 承る=うけたまわる(uketamawaru)), it's still a synthetic language that can have very large conjugations (which have to be written in hiragana) and has a very unfavorable ratio of amount of meaning:syllables (partially mitigated by the use of Chinese characters).